Apparatus for welding a stud to a workpiece

ABSTRACT

Apparatus is presented herein for controlling a stud welding gun for welding a stud to a workpiece. The stud welding gun has a gun body and a gun shaft mounted to the body in such a manner to permit reciprocal movement of the shaft relative to the body. The shaft has a distal end having means for carrying a stud to be welded to the workpiece. A linear motor is carried by the gun for directly driving the shaft in forward and reverse directions between fully extended and fully retracted positions relative to the gun body. A floating point detector provides a control signal when the stud engages the workpiece as the shaft is being driven in a forward direction to establish a reference position of the gun body relative to the workpiece. A control responds to the floating point signal for controlling the motor for driving the shaft in the reverse direction to a retracted lift position away from the workpiece and in the forward plunging direction to a plunge position causing the stud to engage the workpiece to achieve welding.

This is a continuation of copending application Ser. No. 08/009,864filed on Jan. 27, 1993, now U.S. Pat. No. 5,252,802.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention is directed to the art of welding a stud to aworkpiece and, more particularly, to improvements in a stud welding gun.

2. Prior Art

A typical stud welding system includes a power supply, control circuitryand a stud welding gun. Such a welding gun includes a gun body and a gunshaft. The shaft is mounted to the gun body for reciprocal movement withrespect thereto between fully extended and fully retracted positions.The gun shaft has a distal end having means, such as a chuck, forcarrying a stud to be welded to a workpiece. Typically, the gun is ahand held device and includes a member known as a foot which is fixed toand extends forwardly of the gun body and is placed up against aworkpiece to which a stud is to be welded. Thereafter, the operatoractuates a trigger which energizes an electric solenoid which causes theshaft to be retracted to a lift position while appropriate circuitrysupplies welding current to the shaft and the stud. After an arc hasbeen established, the control circuitry deactuates the solenoid and theshaft is driven by a spring so as to plunge toward and cause the stud toengage the workpiece, at which time welding therebetween takes place. Anapparatus for accomplishing the foregoing is disclosed in the U.S.patent to P. A. Glorioso U.S. Pat. No. 5,030,815.

In my previous U.S. Pat. No. 5,070,226 there is disclosed a stud weldinggun similar to that as described above but which does not employ asolenoid for lifting the gun shaft and a spring for, uponde-energization of the solenoid, causing the shaft to plunge toward theworkpiece. Instead, that patent discloses a gun employing a rotarystepper motor interconnected between the gun body and the gun shaft. Themotor has an associated rotary to linear converter which serves to drivethe gun shaft relative to the gun body in forward and reversedirections. Thus, the shaft may be driven in a reverse direction to alift position and thereafter driven in a forward or plunge direction tocause a stud to engage a workpiece to achieve a weld therebetween.

The prior art typically employs a foot on a hand held welding gun toassist in initially positioning the gun relative to a workpiece prior tocommencing a welding cycle. If such a foot is not employed, as in anautomatic welding machine system, difficulties may arise in initializingthe system prior to a welding cycle. That is, a starting or referenceposition of the welding gun relative to the workpiece may be needed.

Moreover, the prior art described above does not provide for a linearmotor for directly driving the gun shaft in forward and reversedirections. Instead, the welding gun disclosed in my aforesaid patentdiscloses a rotary to linear converter in conjunction with a rotarystepper motor for driving the shaft in forward and reverse directions.

SUMMARY OF THE INVENTION

The invention contemplates the provision of apparatus for welding a studto a workpiece which employs a stud welding gun having a gun body, wherea gun shaft is mounted to the body in such a manner to permit reciprocalmovement of the shaft relative to the body. The shaft has a distal endthat has means for carrying a stud.

It has been determined that a linear motor for directly driving the gunshaft in forward and reverse directions can achieve higher speed andlonger life than a rotary stepper motor.

Consequently, in accordance with one aspect of the present invention,the gun incorporates a linear motor for directly driving the shaft inforward and reverse directions between respective fully extended andfully retracted positions relative to the gun body. A controller servesto control the linear motor for driving the shaft in a reverse directionto a retracted lift position spaced away from the workpiece and then ina forward plunging direction to a plunge position causing the stud toengage the workpiece to achieve welding therebetween.

In accordance with another aspect of the present invention, a floatingpoint detector provides a floating point signal when the stud engagesthe workpiece as the shaft is being driven in a forward direction tothereby define a reference position of the gun body relative to theworkpiece. A controller responds to the floating point signal forcontrolling the motor for driving the shaft to a retracted lift positionaway from the workpiece and then driving the shaft in a forward plungingdirection to a plunge position during which the stud is caused to engagethe workpiece to achieve welding therebetween.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features and advantages of the present inventionwill become more apparent to those skilled in the art, to which thepresent invention relates, from a reading of the following descriptionof a preferred embodiment with reference to the accompanying drawings,in which:

FIG. 1 is a schematic-block diagram illustration of a control circuit,in accordance with the present invention, for use with the welding gunshown in FIGS. 2 and 3;

FIG. 2 is a longitudinal cross-sectional view of a welding gun showingthe welding gun shaft in its extended position;

FIG. 3 is a view similar to that of FIG. 2 but showing the welding gunshaft in its retracted position;

FIG. 4 is a graphical illustration showing stud position with respect totime and which is useful in describing the operation of the invention;and

FIG. 5 is a schematic illustration of the linear motor employed herein.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Reference is now made to FIGS. 1, 2 and 3 which illustrate apparatus forwelding a stud S to a workpiece WP (see FIG. 3) and which employs a studwelding gu 10 having a gun body 12 and a gun shaft 14 mounted to thebody in such a manner to permit reciprocal movement of the shaftrelative to the body. The shaft 14 has a distal end which carries achuck 16, of conventional design, which, in turn, is adapted to carrystud S. As is conventional, the chuck 16, in cross section, includes acoaxial array of spaced apart fingers which are designed to resilientlybear against and hold the stud S in place at the distal end of theshaft. Also, as is conventional in the art, the chuck 16 is inelectrical connection with the shaft 14 to a suitable power supply PSwhich, under control of a microcomputer MC, supplies a pilot arc currentand then a main current to the shaft and, thence, to the stud S toachieve welding, in a known manner.

A linear motor 20 is carried by the gun 10 and serves to directly drivethe shaft 14 in forward F and reverse R directions between a fullyextended position 22 and a fully retracted position 24 relative to thegun body. These positions are shown in FIGS. 2 and 3. The linear motor20 will be described in greater detail hereinafter.

An encoder 26 is associated with the motor 20 and serves to provide aposition signal representative of the position of shaft 14 relative tothe body 12 during operation of the linear motor. This position signalis communicated to the microcomputer MC for use in controlling theoperation of the motor. As will be described in greater detailhereinafter, the microcomputer, under program control, operates to drivethe motor to a retracted lift position away from the workpiece duringwhich a pilot arc current and then a main current are applied to thechuck 16. Thereafter, the motor is operated to drive the stud in aforward plunging direction to a plunge position during which the studengages the workpiece to achieve a weld therewith.

Having briefly presented an overview of the invention, attention isdirected to FIGS. 1-5 in conjunction with the following more detaileddescription of the preferred embodiment of the invention. Initially, thestud welding gun 10 is positioned so that its distal end, with the studS in place, is located close to the workpiece WP. The distance betweenthe stud S and the workpiece should be within the range of the forwardmovement of the shaft 14 from its fully retracted position 24 to itsfully extended position 22. This may be achieved by operating a positionmotor 30 which is mechanically connected to the gun body 12 for drivingthe gun body in forward or reverse directions relative to the workpiece.The operator may directly operate the position motor 30 or the operatormay actuate a suitable start control 32 which actuates the microcomputerMC and which, in turn, operates the position motor 30 by way of asuitable motor control circuit 41. This will cause the position motor todrive the gun body 12 in a direction so that the stud S is positionedproximate to (but not contacting) the workpiece WP. Alternatively, theworkpiece WP may be moved relative to the gun body. Also, the gun may behand held and positioned by hand relative to the workpiece.

At this point, the operator may supply an input, as by the start control32, to the microcomputer indicating that a weld operation may commence.Before a weld cycle takes place, the microcomputer will, under programoperation, operate the motor 20 commencing at time T1 (see FIG. 4) todrive shaft 14 in a forward direction in order to locate the position ofthe workpiece WP relative to the gun body 12. As the shaft 14 is drivenin a forward direction, its position relative to the gun body iscommunicated to the microcomputer by way of the encoder 26. The speed ofshaft movement may be on the order of 5 inches per second. When the studS makes engagement with the workpiece WP at time T2 (see FIG. 4), thelocation of the workpiece relative to the gun body is known to themicrocomputer. The point in time that the stud engages the workpiece(time T2 in FIG. 4) is detected by a floating point detector 40 whichprovides a floating point signal to the microcomputer. In FIG. 1, thedetector 40 is characterized as being a simple normally open switchwhich, upon closure, connects a B+ voltage to the microcomputer toindicate that the stud is in contact with the workpiece. In FIG. 3, theB+ voltage source is shown as being electrically connected to the chuck16 which, in turn, is in electrical communication with the stud S. Oncethe stud engages the metal workpiece WP, a circuit is completed at timeT2 (see FIG. 4) between the B+ voltage supply source and the detector 40so that the floating point signal may be transmitted to themicrocomputer MC. Detector 40 may include an optical coupler.

With the floating point established, the microcomputer MC is nowconditioned to commence a weld cycle WC. During each weld cycle WC, themicrocomputer MC, under program control, initiates a hold period fromtime T2 to time T3 during which a pilot current is applied. No arc isestablished at this point. The duration of the interval from time T2 totime T3 may be on the order to 10 milliseconds, for example.

At time T3, the computer initiates a lift operation during which thestud is lifted in a reverse direction away from the workpiece WP to alift position 50 (see FIG. 4). The lift position 50 is a known distancefrom the hold position 48 and this is programmed into the microcomputer.The speed of movement from time T3 to time T4 is not critical but shouldbe sufficiently fast to minimize the time between weld cycles. Duringthis lift period, the gap between the stud and the workpiece increasesand the pilot current is sufficient to establish and maintain a pilotarc. The time duration of the lift period from time T3 to time T4 may beon the order of 8 to 10 milliseconds, for example.

When the stud has been retracted to the lift position 50, it is heldthere for a time period which, for example, may be on the order of 20milliseconds. During this time, the computer is programmed to apply themain welding current to the stud to strengthen the arc and thusestablish the main arc. At the completion of the lift period at time T5,the computer initiates a plunge stroke by activating the motor 20 sothat the motor is driven in a forward plunging direction causing thestud to plunge toward and engage the workpiece WP. The plunge strokeextends from time T5 to time T6. At time T6, the stud has arrived at thefinal plunge position 52. The final plunge position is maintained fromtime T6 to time T7 which is sufficient time for the weld to solidify.This period may be on the order of 5 milliseconds, for example. Theplunge velocity is controlled by the computer so that the weld currentis turned off at the point in time that the stud makes engagement withthe workpiece. At time T7, the computer controls the motor 20 to drivethe chuck to its fully retracted position 24 (see FIG. 3) which takesplace at time T8. The stud is now welded to the workpiece and pulls outof the chuck as the chuck retracts. A new stud is now loaded into thechuck 16 either manually or automatically through equipment not shown.The welding gun is now in condition to commence another cycle ofoperation.

As shown in FIG. 2, 3 and 5, the linear motor 20 includes coils A and Bwhich are connected together in series but are reverse wound relative toeach other so that their respective magnetic end poles are reversed (aswill be discussed in greater detail hereinafter). The two coils arewrapped about an elongated cylindrical bobbin 60 which coaxiallysurrounds and slidably receives a piston shaped permanent magnet 62. Thebobbin is constructed of non-magnetic material, such as anodizedaluminum, to isolate the coil from the magnet, both electrically andmagnetically. The permanent magnet may, for example, have a south pole Sat its left end and a north pole N at its right end, as indicated by themarkings in FIGS. 2 and 3. The magnet has a pair of end pieces or disks64 and 66 having diameters slightly greater than the rest of the magnet.Disks 64 and 66 are constructed of mild ferromagnetic, low carbon (soft)steel. However, the diameters of these end pieces are sufficiently lessthan the inner diameter of the bobbin 60 that the magnet is slidablyreceived so that it may reciprocate along its axis in forward (right)and reverse (left) directions. The bobbin 60 is provided with radiallyextending flanges or dividers 70, 72 and 74 which serve to provide areasfor receiving coils A and B while keeping the coils separated from eachother during the operation. The coils A and B are surrounded by an outercasing or housing 80, which may take the form of an elongated tubularsteel casing. The housing 80 is constructed of mild ferromagnetic, lowcarbon (soft) steel. At its left end, as viewed in FIGS. 2 and 3, thesteel casing is provided with a disk-shaped cap or end plate 82 having acentral bore 84 therein for receiving a portion of the proximal end ofshaft 14 when the shaft is retracted to its fully retracted position, asshown in FIG. 3. The end plate 82 is constructed of non-magneticmaterial, such as aluminum.

The shaft 14 is constructed of non-magnetic material, such as stainlesssteel or aluminum. At its proximal end, the shaft has a reduced diameterportion 90 which extends through a suitable bore in the permanentmagnet. The proximal end of shaft portion 90 is threaded and receives anut 92 to secure the shaft 14 to the magnet 62 while keeping end plates64 and 66 in place.

As shown in FIGS. 2 and 3, a disk-shaped bearing plate 96 is interposedbetween nut 92 and end piece 64. Bearing plate 96 is constructed ofnon-magnetic material, such as aluminum or plastic. This bearing plate96 has an outer diameter just slightly less than that of the innerdiameter of the bobbin 60 and provides a bearing surface for engagingthe inner surface of the bobbin during reciprocal movement of thepermanent magnet.

The end plate 82 serves as a stop to prevent further reverse movement ofthe magnet when the end plate is struck by the bearing plate 96. On theright side of the housing 80, there is provided an annular sleeve 100which coaxially surrounds the shaft 14. Sleeve 100 is constructed ofnon-magnetic material, such as aluminum. This sleeve receives an annularfront bearing 102 which coaxially surrounds and slidably receives theshaft 14 during its reciprocal movement between fully extended and fullyretracted positions. Bearing 102 is constructed of non-magneticmaterial, such as plastic. The sleeve 100 is necked in somewhat andsecured to housing 80 by means of a screw 104. A similar screw 105secures the end piece 82 to the housing 80. The left end 106 of sleeve100 serves as a stop to prevent further forward movement of thepermanent magnet when end plate 66 engages the left end 106 of thesleeve.

A slot is provided in bearing 102 to receive a stationary portion of theencoder 26 (see also FIG. 1). This encoder serves to provide a positionsignal indicating the position of the shaft 14 relative to the gun body12 while the shaft is displaced between its fully retracted position asshown in FIG. 3 and its fully extended position as shown in FIG. 2. Thisencoder may take the form of a potentiometer such as that provided in myprevious U.S. Pat. No. 5,070,226. Preferably, the encoder takes the formof a relative movement optical encoder wherein the stationary portion asviewed in FIGS. 2 and 3 includes optical transmitter means, such aslight emitting diodes, and optical receiver means, such as phototransistors. In such case, light may be reflected off markings on theshaft. Alternatively, the shaft may carry an encoder card havingsuitable optical markings thereon. Such an encoder will provide positionand direction of movement information to inform the microcomputer MC theposition of the shaft relative to the gun body as its moves in forwardand reverse directions.

At its distal end, the shaft 14 has a bore which carries a stop 120against which the chuck 16 abuts. The chuck 16 is held in place by meansof a nut 122 which surrounds the chuck 16 and is provided with internalthreads to make threaded engagement with external threads on shaft 14.

Attention is now directed to FIG. 5 which presents a schematicillustration of the linear motor 20 showing the permanent magnet 62 indotted lines in its fully retracted position (see FIG. 3). As notedhereinbefore, the coils A and B are connected together in series but arereverse wound relative to each other so that their magnetic poles arereversed. Thus, if current I1 is caused to flow in the direction asindicated in FIG. 5, then this current will flow through the seriesconnected coils causing coil A to have a south pole and a north polewith the south pole being at the left as designated S1 and the northpole being at the right end of the coil and designated N1. Similarly, asthe current flows through coil B, it will have a magnetic field suchthat the left end may be a north pole, designated N1, and the right endof the coil may be a south pole, designated S1.

Initially, with the permanent magnet 62 having its south pole S alignedwith the south pole S1 of the coil A, the permanent magnet will berepelled by the south pole of the coil. The magnet is prevented frommoving to the left because of its engagement with end piece 82. Thisrepelling force will cause the magnet to move toward the right in aforward direction F. As the permanent magnet continues to move in theforward direction F, the south pole S1 of coil B will attract the northpole N of the magnet. Also, the north pole N1 of coil B will attract thesouth pole S of the magnet, causing the permanent magnet to continue tomove toward the right in a forward direction F. This forward movementwill continue until the magnet is brought to a halt by the end polepiece 66 on the magnet striking the left end 106 of the sleeve 100. Atthis point, the permanent magnet will be moved all the way to the rightin FIG. 5 so that the north pole N of the magnet is aligned with theright end of coil B and the south pole of the magnet is aligned with theleft end of coil B. At this point, the computer reverses the flow ofcurrent so that current I2 flows in the opposite direction through thecoils A and B in the direction as indicated in FIG. 5. This causes coilsA and B to have their magnetic end poles reversed as indicated bydesignations N2 and S2 just below the coils in FIG. 5. Consequently, ascurrent I2 flows through the coils in the direction as noted, the northpole N of the permanent magnet 62 is repelled by the north pole N2 ofcoil B and the permanent magnet moves to the left as viewed in FIG. 5 inthe reverse direction R. As the permanent magnet gets about one-half ofthe distance to the coil, the north pole N2 of coil A will attract thesouth pole S of the permanent magnet 62 causing the permanent magnet tocontinue to move toward the left in the reverse direction R and thismovement will continue until bearing 96 on the left end of the magnetstrikes the end piece 82 (which acts as a stop) of the housing.

In the above discussion, movement of the permanent magnet in either theforward direction F or the reverse direction R was terminated by themagnet engaging a stop provided at one end of the gun housing. Themagnet may, however, be stopped at a location intermediate the fullyextended and fully retracted positions represented by the stops. Thus,with reference to FIG. 4, the computer at time T3 commands the weldinggun to retract the shaft so that it is retracted to a lift position 50from the hold position 48. This retraction is a known distance which hasbeen programmed in the computer. Consequently, when the computercommands the linear motor to move the shaft 14 in a reverse direction,current I2 (FIG. 5) will be applied to coils A and B. Since the liftposition 50 is known to the computer, the position signal obtained fromthe encoder 26 is monitored to find the location of the permanent magnetwhich corresponds to the programmed distance from the hold position 48to the lift position 50. At that point, the permanent magnet is stoppedby reversing the current flow from I2 to I1 and monitoring the positionsensed by encoder 26. Some hunting may take place in the event that themagnet overshoots the intended lift position 50. Consequently, as thecomputer controls the welding gun during the welding cycle WC from timeT2 through T7, the computer will monitor the position sensed by theencoder 26 to control the positioning of the gun shaft to correspondwith the lift position 50 and then the plunge position 52 beforecommanding the gun shaft to return to its retracted position at time T8at which the reverse movement of the permanent magnet will be stopped bythe magnet engaging end piece 82.

Although the invention has been described in conjunction with apreferred embodiment, it is to be appreciated that various modificationsmay be made without departing from the spirit and scope of the inventionas defined by the append claims.

Having described the invention, the following is claimed:
 1. Apparatus for welding a stud to a workpiece, comprising:a stud welding gun having a gun body and a gun shaft mounted to said body so as to permit reciprocal movement of said shaft relative to said body, said shaft having a distal end having means for carrying said stud; linear motor means carried by said gun for directly driving said shaft in forward and reverse directions between respective fully extended and fully retracted positions relative to said gun body, said linear motor means including a permanent magnet connected to said shaft for said reciprocal movement in said forward and reverse directions therewith and stationary coil means connected to said gun body and wherein said coil means includes first and second elongated coils coaxially surrounding and radially spaced from said magnet as said magnet moves in said forward and reverse directions, said first and second coils being connected together in series and being reverse wound relative to each other so that when direct current flows therethrough their respective magnetic poles are reversed relative to each other; and control means for controlling said linear motor means for driving said shaft in a reverse direction to a retracted lift position away from said workpiece and in a forward plunging direction to a plunge position causing said stud to engage said workpiece and including circuit means interconnected with said coil means for controlling direct current flow through said coil means in first and second directions to achieve movement of said shaft in said respective forward and reverse directions.
 2. Apparatus as set forth in claim 1 wherein said control means include circuit means for supplying direct current to flow in a first direction through said series connected first and second coils to cause said permanent magnet to move in a said forward direction and for reversing the flow of said direct current so as to flow in a second direction through said series connected first and second coils to cause said permanent magnet to move in a said reverse direction.
 3. Apparatus as set forth in claim 1 wherein said coil means is of a length greater than that of said permanent magnet.
 4. Apparatus as set forth in claim 3 wherein each said first and second coil is of a length greater than that of said permanent magnet.
 5. Apparatus as set forth in claim 1 wherein said permanent magnet is an elongated cylindrical member having an axially-extending bore extending therethrough and wherein said gun shaft has an axially-extending portion which is received by the bore in said permanent magnet. 